The present invention relates to an ink-jet printing method for painting an image on a recording medium with ink drops ejected from ink-jet heads and more particularly relates to an ink-jet printing method wherein the number of cells of a matrix for a single picture element can be varied.
In the case of painting an image with half-tones such as photographs by using an ink-jet printer, it is necessary to be able to express half-tones in at least about sixteen steps of gradations. An ink-jet printing method suitable for expressing half-tones is well known in this art wherein the number of ink dots and their locations to be deposited in a dot matrix having M possible positions on the rows and N possible positions on the columns (which together can be thought of as a cell) for a single picture element are changed in order to produce half-tones. The dot matrix for a single picture element includes a plurality of square cells on each of which a single ink drop is permitted to be deposited. When it is intended to reproduce an image the same as or proportional to an original, a dot matrix having the same number of possible positions on each of the rows and the columns, that is, in which M and N are numerically equal, is used.
An ink-jet printing method wherein a single dot matrix is allocated to respective picture elements allows painting an image with half-tones closely similar to an original and advantageously permits a memory for memorizing the image information to have a small capacity since a picture element to be produced or painted can be made of a large number of ink dots when using a large size of dot matrix having an increased number of cells therein. On the other hand, there is the disadvantage that the image painted will be lowered in resolution. On the contrary, when using a small size of dot matrix having cells reduced in number, the image painted will be improved in resolution since a single picture element is made of a small number of ink dots. However, there is then the disadvantage that the ability to paint an image with half-tones is reduced and a large-capacity memory for memorizing image information is necessary.
In a method which is conventionally employed in consideration of the facts described above, the size of a single dot matrix for a picture element is chosen in accordance with the subject matter so as to paint desirable images; for instance, a large size of dot matrix is chosen when it is necessary to reproduce an image with half-tones closely similar to those of photographs and a small size of dot matrix is chosen when it is necessary to paint characters and line drawings demanding great resolution. However, in an ink-jet printer wherein a recording paper wrapped around a rotary drum is rotated at a fixed speed and ink drops are deposited thereon under the control of signals synchronized with the rotation of the drum so as to paint picture elements one by one, there is the disadvantage that some ink-jet heads will be of no practical use when a small size of dot matrix is chosen in place of a large size of dot matrix, and that it takes a long time to paint a predetermined area.